The present invention relates to a refrigerator and, paticularly, to a closed cycle gas refrigerator for use in cooling, for example, infrared detectors.
FIG. 4 is a cross section of a conventional gas refrigerator of such type as shown in Japanese Patent Publication No. 28980/1979. In this figure, a reference numeral 1 depicts a cylinder in which a piston 2 and a displacer 3 reciprocate with phases different from each other. A cooler 5 is disposed in a compression space 4 between a working surface 2a of the piston 2 and a working surface 3a of the displacer 3. An upper working surface 3b of the displacer 3 forms a border line of an expansion space 6 which forms, together with the compression space 4, a working space. A regenerator 7 disposed in the displacer 3 can be communicated with a working gas below a center hole 8 therethrough and with a working gas above a radial duct 10 through the latter and a center hole 9. The refrigerator is equipped with a freezer 11 as a heat exchanger for exchanging heat between cold working gas and a material to be cooled.
Seals 12 and 13 are provided between the piston 2 and a wall of the cylinder 1 and seals 14 and 15 are provided between the displacer 3 and the cylinder 1.
The piston 2 has a light weight sleeve 16 of non-magnetic or non-magnetized material such as paper or aluminum on a lower portion thereof, on which an armature coil 17 having lead wires 18 and 19 is wound. The lead wires extend through a wall of a housing 20 connected air-tightly to the cylinder 1 and are connected to electric contacts 21 and 22, respectively. The armature coil 17 is able to reciprocate axially in an annular gap 23, in which an armature magnetic field is established. Magnetic flux of this magnetic field extends radially across moving directions of the armature coil.
A static magnetic field is provided by an annular permanent magnet 24 having magnetic poles at its upper and lower ends, an annular disc 25 of soft iron, a cylinder 26 of soft iron and a circular disc 27 of soft iron, which, all together, constitute a closed magnetic circuit.
Further, the sleeve 16, the armature coil 17, the lead wires 18 and 19, the annular gap 23, the annular permanent magnet 24 and the parts 25, 26 and 27 of soft iron constitute a linear motor 28.
In the conventional refrigerator mentioned above, an a.c. power source is connected through the contacts 21 and 22 to the armature coil 17 to supply a power necessary to operate it. An angular .omega..sub.o frequency of the a.c. power source is substantially equal to an angular resonance frequency .omega. of an assembly of the piston 2 and the armature coil 17, which is defined as follows: ##EQU2## where S=area of the working surface 2a of the piston 2
Pm=mean working gas pressure in the working space formed mainly by the compression space 4 and the expansion space 6. PA1 M=total mass of the piston 2, the sleeve 16 and the armature coil 17. PA1 To=ambient absolute temperature. PA1 ac.sub.c =Cp/Cv=(specific heat of the working gas in the compression space 4 at constant pressure)/(specific heat of the working gas in the compression space 4 at constant volume) PA1 Vc=volume of the compression space 4. PA1 Tc=mean operating absolute temperature of the working gas in the compression space 4. PA1 ae.sub.e =Cp/Cv=(specific heat of the working gas in the expansion space 6 at constant pressure)/(specific heat of the working gas in the expansion space 6 at constant volume). PA1 Ve=volume of the expansion space 6. PA1 Te=mean operating absolute temperature of the working gas in the expansion space 6. ##EQU3## where Vw=volume of working gas of the associated heat exchanger. PA1 Tw=mean absolute working gas temperature in the heat exchanger in operation.
An acceptable error between the angular frequencies .omega..sub.o and .omega. may be 10% or smaller.
In operation, when the a.c. power source having angular frequency .omega..sub.o which is substantially equal to .omega. defined by the equation (1) is connected to the contacts 21 and 22, the armature coil 17 is subjected to an axial Lorentz force due to the presence of the permanent magnetic field in the gap 23. As a result, the assembly of the piston 2, the sleeve 16 and the armature 17 resonates and vibrates axially. The vibration of the piston 2 causes a periodic pressure variation to occur in the working gas filling the working space formed by the compression space 4, the expansion space 6, the cooler 5, the regenerator 7, the center holes 8 and 9, the radial duct 10 and the freezer 11 and a resultant change of flow rate of gas through the regenerator 7 causes a periodically alternating driving force to be exerted on the displacer 3. Thus, the displacer 3 including the regenerator 7 is caused to reciprocated axially in the cylinder 1 at the same frequency as that of the piston 2 with different phase therefrom.
When the phase difference is kept constant suitably, the working gas in the working space repeats a thermodynamic cycle known as the "Inverse Stirling Cycle" and generates cold production mainly in the expansion gap 6 and the freezer 11.
The "Inverse Stirling Cycle" and the principle of generation of and cold states thereby is described in detail in "Cryocoolers", G. Walker, Plenum Press, New York, 1983, pp 117-123. In this specification, the principle will be described briefly.
The working gas in the compression space 4 which has been compressed by the piston 2 and heated thereby is cooled while flowing through the cooler 5 and flows into the hole 8 and then into the regenerator 7 in which it is further cooled by low temperature heat accumulated in a preceding half cycle. Then, it flows through the center hole 9, the radial duct 10 and the freezer 11 into the expansion space 6. After a substantial amount of the working gas is flown into the expansion space 6, an expansion stroke of the piston 2 is started and results in a lower temperature state in the expansion space 6. Then, the working gas flows in the reverse direction while giving the low temterature heat to the regenerator 7 into the compression space 4. In this cycle, the cold working gas absorbs heat of an external substance while passing through the freezer 11 to cool it. After a substantial amount of the working gas is returned to the compression space 4, the compression stroke is started again. The "Inverse Stirling Cycle" is completed in this manner. For more detail, the above mentioned article should be referred to.
In the conventional refrigerator, however, the angular resonance frequency defined by the equation (1) tends to be changed by leakage of the working gas through the seals, polytropic compression/expansion of the working gas and/or a use of mechanical springs of large spring constant. Therefore, it is difficult to constitute an efficient refrigerator.